<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="en" dir="ltr" class="client-nojs">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Definitions and ODR - cppreference.com</title>
<meta charset="UTF-8">
<meta name="generator" content="MediaWiki 1.21.2">


<link rel="shortcut icon" href="../../../favicon.ico">



<link rel="stylesheet" href="../../../common/ext.css">
<meta name="ResourceLoaderDynamicStyles" content="">
<link rel="stylesheet" href="../../../common/site_modules.css">
<style>a:lang(ar),a:lang(ckb),a:lang(fa),a:lang(kk-arab),a:lang(mzn),a:lang(ps),a:lang(ur){text-decoration:none}#toc{display:none}.editsection{display:none}
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-css:7:472787eddcf4605d11de8c7ef047234f */</style>

<script src="../../../common/startup_scripts.js"></script>
<script>if(window.mw){
mw.config.set({"wgCanonicalNamespace":"","wgCanonicalSpecialPageName":false,"wgNamespaceNumber":0,"wgPageName":"cpp/language/definition","wgTitle":"cpp/language/definition","wgCurRevisionId":69683,"wgArticleId":13180,"wgIsArticle":true,"wgAction":"view","wgUserName":null,"wgUserGroups":["*"],"wgCategories":["Todo with reason"],"wgBreakFrames":false,"wgPageContentLanguage":"en","wgSeparatorTransformTable":["",""],"wgDigitTransformTable":["",""],"wgDefaultDateFormat":"dmy","wgMonthNames":["","January","February","March","April","May","June","July","August","September","October","November","December"],"wgMonthNamesShort":["","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"wgRelevantPageName":"cpp/language/definition","wgRestrictionEdit":[],"wgRestrictionMove":[]});
}</script><script>if(window.mw){
mw.loader.implement("user.options",function(){mw.user.options.set({"ccmeonemails":0,"cols":80,"date":"default","diffonly":0,"disablemail":0,"disablesuggest":0,"editfont":"default","editondblclick":0,"editsection":0,"editsectiononrightclick":0,"enotifminoredits":0,"enotifrevealaddr":0,"enotifusertalkpages":1,"enotifwatchlistpages":0,"extendwatchlist":0,"externaldiff":0,"externaleditor":0,"fancysig":0,"forceeditsummary":0,"gender":"unknown","hideminor":0,"hidepatrolled":0,"imagesize":2,"justify":0,"math":1,"minordefault":0,"newpageshidepatrolled":0,"nocache":0,"noconvertlink":0,"norollbackdiff":0,"numberheadings":0,"previewonfirst":0,"previewontop":1,"quickbar":5,"rcdays":7,"rclimit":50,"rememberpassword":0,"rows":25,"searchlimit":20,"showhiddencats":0,"showjumplinks":1,"shownumberswatching":1,"showtoc":0,"showtoolbar":1,"skin":"cppreference2","stubthreshold":0,"thumbsize":2,"underline":2,"uselivepreview":0,"usenewrc":0,"watchcreations":0,"watchdefault":0,"watchdeletion":0,
"watchlistdays":3,"watchlisthideanons":0,"watchlisthidebots":0,"watchlisthideliu":0,"watchlisthideminor":0,"watchlisthideown":0,"watchlisthidepatrolled":0,"watchmoves":0,"wllimit":250,"variant":"en","language":"en","searchNs0":true,"searchNs1":false,"searchNs2":false,"searchNs3":false,"searchNs4":false,"searchNs5":false,"searchNs6":false,"searchNs7":false,"searchNs8":false,"searchNs9":false,"searchNs10":false,"searchNs11":false,"searchNs12":false,"searchNs13":false,"searchNs14":false,"searchNs15":false,"gadget-ColiruCompiler":1});;},{},{});mw.loader.implement("user.tokens",function(){mw.user.tokens.set({"editToken":"+\\","patrolToken":false,"watchToken":false});;},{},{});
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-js:7:ca03345b1e2c4d90a25d968753a73b92 */
}</script>
<script>if(window.mw){
mw.loader.load(["mediawiki.page.startup","mediawiki.legacy.wikibits","mediawiki.legacy.ajax"]);
}</script>
<style type="text/css">/*<![CDATA[*/
.source-cpp {line-height: normal;}
.source-cpp li, .source-cpp pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for cpp
 * CSS class: source-cpp, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.cpp.source-cpp .de1, .cpp.source-cpp .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.cpp.source-cpp  {font-family:monospace;}
.cpp.source-cpp .imp {font-weight: bold; color: red;}
.cpp.source-cpp li, .cpp.source-cpp .li1 {font-weight: normal; vertical-align:top;}
.cpp.source-cpp .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.cpp.source-cpp .li2 {font-weight: bold; vertical-align:top;}
.cpp.source-cpp .kw1 {color: #0000dd;}
.cpp.source-cpp .kw2 {color: #0000ff;}
.cpp.source-cpp .kw3 {color: #0000dd;}
.cpp.source-cpp .kw4 {color: #0000ff;}
.cpp.source-cpp .co1 {color: #909090;}
.cpp.source-cpp .co2 {color: #339900;}
.cpp.source-cpp .coMULTI {color: #ff0000; font-style: italic;}
.cpp.source-cpp .es0 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es1 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es2 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es3 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es4 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es5 {color: #008000; font-weight: bold;}
.cpp.source-cpp .br0 {color: #008000;}
.cpp.source-cpp .sy0 {color: #008000;}
.cpp.source-cpp .sy1 {color: #000080;}
.cpp.source-cpp .sy2 {color: #000040;}
.cpp.source-cpp .sy3 {color: #000040;}
.cpp.source-cpp .sy4 {color: #008080;}
.cpp.source-cpp .st0 {color: #008000;}
.cpp.source-cpp .nu0 {color: #000080;}
.cpp.source-cpp .nu6 {color: #000080;}
.cpp.source-cpp .nu8 {color: #000080;}
.cpp.source-cpp .nu12 {color: #000080;}
.cpp.source-cpp .nu16 {color:#000080;}
.cpp.source-cpp .nu17 {color:#000080;}
.cpp.source-cpp .nu18 {color:#000080;}
.cpp.source-cpp .nu19 {color:#000080;}
.cpp.source-cpp .ln-xtra, .cpp.source-cpp li.ln-xtra, .cpp.source-cpp div.ln-xtra {background-color: #ffc;}
.cpp.source-cpp span.xtra { display:block; }

/*]]>*/
</style>
<!--[if lt IE 7]><style type="text/css">body{behavior:url("/mwiki/skins/cppreference2/csshover.min.htc")}</style><![endif]-->
</head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_definition skin-cppreference2 action-view cpp-navbar">
        <!-- header -->
        
        <!-- /header -->
        <!-- content -->
        <div id="cpp-content-base">
            <div id="content">
                <a id="top"></a>
                <div id="mw-js-message" style="display:none;"></div>
                                <!-- firstHeading -->
                <h1 id="firstHeading" class="firstHeading">Definitions and ODR</h1>
                <!-- /firstHeading -->
                <!-- bodyContent -->
                <div id="bodyContent">
                                        <!-- tagline -->
                    <div id="siteSub">From cppreference.com</div>
                    <!-- /tagline -->
                                        <!-- subtitle -->
                    <div id="contentSub"><span class="subpages">&lt; <a href="../../cpp.html" title="cpp">cpp</a>‎ | <a href="../language.1.html" title="cpp/language">language</a></span></div>
                    <!-- /subtitle -->
                                                            <!-- bodycontent -->
                    <div id="mw-content-text" lang="en" dir="ltr" class="mw-content-ltr">
<div class="t-navbar" style="">
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head">
<a href="../../cpp.html" title="cpp"> C++</a><div class="t-navbar-menu"><div>
<div><table class="t-nv-begin" cellpadding="0" style="line-height:1.1em;">
<tr class="t-nv"><td colspan="5"> <a href="../language.1.html" title="cpp/language"> Language</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../header.html" title="cpp/header"> Standard library headers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concept.html" title="cpp/concept"> Concepts</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html" title="cpp/utility"> Utilities library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../string.html" title="cpp/string"> Strings library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../container.html" title="cpp/container"> Containers library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm.html" title="cpp/algorithm"> Algorithms library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../iterator.html" title="cpp/iterator"> Iterators library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../numeric.html" title="cpp/numeric"> Numerics library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../io.html" title="cpp/io"> Input/output library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../locale.html" title="cpp/locale"> Localizations library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../regex.html" title="cpp/regex"> Regular expressions library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../atomic.html" title="cpp/atomic"> Atomic operations library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread"> Thread support library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
</table></div>
<div></div>
</div></div>
</div>
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head"><a href="../language.1.html" title="cpp/language"> C++ language</a></div>
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head">
<a href="basic_concepts.html" title="cpp/language/basic concepts"> Basic Concepts</a><div class="t-navbar-menu"><div>
<div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../keywords.html" title="cpp/keyword"> language keywords</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="translation_phases.html" title="cpp/language/translation phases"> phases of translation</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../comment.html" title="cpp/comment"> comments </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="main_function.html" title="cpp/language/main function"> the main() function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="name.html" title="cpp/language/identifiers"> names and identifiers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="incomplete_type.html" title="cpp/language/type"> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="types.html" title="cpp/language/types"> arithmetic types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="objects.html" title="cpp/language/object"> objects</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="scope.html" title="cpp/language/scope"> scope</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lifetime.html" title="cpp/language/lifetime"> object lifetime</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="storage_duration.html" title="cpp/language/storage duration"> storage duration and linkage</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> definitions and ODR</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lookup.html" title="cpp/language/lookup"> name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="memory_model.html" title="cpp/language/memory model"> memory model</a> </td></tr>
</table></div>
</div>
<div></div>
</div></div>
</div>
<div class="t-navbar-sep"> </div>
</div>
<p><i>Definitions</i> are <a href="declarations.html" title="cpp/language/declarations">declarations</a> that fully define the entity introduced by the declaration. Every declaration is a definition, except for the following:
</p>
<div class="t-li1">
<span class="t-li">1)</span> Any declaration with an <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">extern</span></span></span> <a href="storage_duration.html" title="cpp/language/storage duration">storage class specifier</a> or with a <a href="language_linkage.html" title="cpp/language/language linkage">language linkage</a> specifier (such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">extern</span> <span class="st0">"C"</span></span></span>) without an initializer
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="kw4">const</span> <span class="kw4">int</span> a<span class="sy4">;</span> <span class="co1">// declares, but doesn't define a</span>
<span class="kw4">extern</span> <span class="kw4">const</span> <span class="kw4">int</span> b <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// defines b</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">2)</span> Function declaration without a function body
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define f</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">3)</span> Parameter declaration in a function declaration that isn't a definition
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define f and x</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// defines f and x</span>
     <span class="kw1">return</span> x<span class="sy2">+</span>a<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">4)</span> Declaration of a <a href="static.html" title="cpp/language/static">static data member</a> inside a class definition
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>    <span class="co1">// defines S</span>
    <span class="kw4">int</span> n<span class="sy4">;</span>        <span class="co1">// defines S::n</span>
    <span class="kw4">static</span> <span class="kw4">int</span> i<span class="sy4">;</span> <span class="co1">// declares, but doesn't define S::i</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> S<span class="sy4">::</span><span class="me2">i</span><span class="sy4">;</span> <span class="co1">// defines S::i</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">5)</span> Declaration of a class name (by <a href="class.html%23Forward_declaration" title="cpp/language/class">forward declaration</a> or by the use of the elaborated type specifier in another declaration)
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S<span class="sy4">;</span> <span class="co1">// declares, but doesn't define S</span>
<span class="kw1">class</span> Y f<span class="br0">(</span><span class="kw1">class</span> T p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define Y and T (and also f and p)</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">6)</span> Declaration of a <a href="template_parameters.html" title="cpp/language/template parameters">template parameter</a>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span> <span class="co1">// declares, but doesn't define T</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">7)</span> <a href="typedef.html" title="cpp/language/typedef">typedef</a> declaration
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">typedef</span> S S2<span class="sy4">;</span> <span class="co1">// declares, but doesn't define S2 (S may be incomplete)</span></pre></div></div>
<ul><li> <span class="t-mark-rev t-since-cxx11">(C++11)</span> <a href="type_alias.html" title="cpp/language/type alias">alias-declaration</a>
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">using</span> S2 <span class="sy1">=</span> S<span class="sy4">;</span> <span class="co1">// declares, but doesn't define S2 (S may be incomplete)</span></pre></div></div>
<ul><li> <span class="t-mark-rev t-since-cxx11">(C++11)</span> <a href="enum.html" title="cpp/language/enum">Opaque declaration</a> of an enumeration.
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw2">enum</span> Color <span class="sy4">:</span> <span class="kw4">int</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define Color</span></pre></div></div>
<ul><li> <a href="using_declaration.html" title="cpp/language/using declaration">using-declaration</a>
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">using</span> N<span class="sy4">::</span><span class="me2">d</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define d</span></pre></div></div>
<ul>
<li> <a href="namespace.html%23Using-directives" title="cpp/language/namespace">using-directive</a> (does not define any entities)
</li>
<li> <span class="t-mark-rev t-since-cxx11">(C++11)</span> <a href="static_assert.html" title="cpp/language/static assert">static_assert</a> declaration (does not define any entities)
</li>
<li> <span class="t-mark-rev t-since-cxx11">(C++11)</span> <a href="declarations.html" title="cpp/language/declarations">Attribute declaration</a> (does not define any entities)
</li>
<li> <a href="declarations.html" title="cpp/language/declarations">Empty declaration</a> (does not define any entities)</li>
</ul>
</div>

<p>Although <a href="asm.html" title="cpp/language/asm">asm declaration</a> does not define any entities as well, it is a definition.
</p>
<p>Where necessary, the compiler may implicitly define the <a href="default_constructor.html" title="cpp/language/default constructor">default constructor</a>, <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a>, <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a>, <a href="as_operator.html" title="cpp/language/as operator">copy assignment operator</a>, <a href="move_operator.html" title="cpp/language/move operator">move assignment operator</a>, and the <a href="destructor.html" title="cpp/language/destructor">destructor</a>.
</p>
<p>If the definition of any object results in an object of <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete type</a>, the program is ill-formed.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: One Definition Rule">edit</a>]</span> <span class="mw-headline" id="One_Definition_Rule">One Definition Rule</span>
</h3>
<p>Only one definition of any variable, function, class type, enumeration type, or template is allowed in any one translation unit.
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: ODR-use: rest of 3.2 and templates </td>
</tr></table>

<!-- 
NewPP limit report
Preprocessor visited node count: 1142/1000000
Preprocessor generated node count: 5407/1000000
Post‐expand include size: 25830/2097152 bytes
Template argument size: 4194/2097152 bytes
Highest expansion depth: 17/40
Expensive parser function count: 0/100
-->

<!-- Saved in parser cache with key mwiki1-mwiki_en_:pcache:idhash:13180-0!*!0!*!*!*!* and timestamp 20140419151423 -->
</div>                    <!-- /bodycontent -->
                                        <!-- printfooter -->
                    <div class="printfooter">
                    Retrieved from "<a href="http%3A//en.cppreference.com/mwiki/index.php">http://en.cppreference.com/mwiki/index.php?title=cpp/language/definition&amp;oldid=69683</a>"                    </div>
                    <!-- /printfooter -->
                                                            <!-- catlinks -->
                    <div id="catlinks" class="catlinks"><div id="mw-normal-catlinks" class="mw-normal-catlinks">
<a href="http%3A//en.cppreference.com/w/Special%3ACategories" title="Special:Categories">Category</a>: <ul><li><a href="http%3A//en.cppreference.com/w/Category%3ATodo_with_reason" title="Category:Todo with reason">Todo with reason</a></li></ul>
</div></div>                    <!-- /catlinks -->
                                                            <div class="visualClear"></div>
                    <!-- debughtml -->
                                        <!-- /debughtml -->
                </div>
                <!-- /bodyContent -->
            </div>
        </div>
        <!-- /content -->
        <!-- footer -->
        
        <!-- /footer -->
        <script>if(window.mw){
mw.loader.state({"site":"loading","user":"missing","user.groups":"ready"});
}</script>
<script src="../../../common/skin_scripts.js"></script>
<script>if(window.mw){
mw.loader.load(["mediawiki.action.view.postEdit","mediawiki.user","mediawiki.page.ready","mediawiki.searchSuggest","mediawiki.hidpi","ext.gadget.ColiruCompiler"], null, true);
}</script>
<script src="../../../common/site_scripts.js"></script>

<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-2828341-1']);
_gaq.push(['_setDomainName', 'cppreference.com']);
_gaq.push(['_trackPageview']);
</script><!-- Served in 0.083 secs. -->
	</body>
<!-- Cached 20140419160955 -->
</html>
